Different Req_fuel: running rich

sebch · Post by **sebch** » Wed Mar 22, 2006 2:06 am

Hi,
Two days ago I changed my FPR that was leaking fhel through it's vac line to the intake manifold (rest in peace!). The unit I found in my stuff is rated at 3 bars as opposed to 2.5 for the dead unit. I'll save you all the math but my injectors were promoted from 18.3 to 20 lbs/hr. This means that my req_fuel went down from 17.6 to 16.0. So I changed the value in the constants to reflect the mod but I'm running rich. My tuning was still not perfect but quite good at cruising speeds, but now with the EGO corr turned on (limit is 30% to datalog), It sticks between 80 and 100% with an occasional peak at 110 or 120% (less than 1%) of the total time.

Is there a step I missed in altering my settings following the FPR change?

This is board V2.2, code 3.0.

Thanks, SÃ©b

Jedrik · Post by **Jedrik** » Wed Mar 22, 2006 2:14 am

There are a few options. You can set req fuel even lower than your calculations. Really that is what it is, a scale factor. If you prefer to keep the theoreticaly correct req_fuel you can scale the VE table:

In MegaTune, go to Tables:VE Table 1:Tools:VE Specific:Reset Reqfuel
Experiment to see how it works.

Or you could use Table Transform.

sebch · Post by **sebch** » Wed Mar 22, 2006 6:10 am

OK, Jedrik, thanks for the answer, I'll try a few things as you suggest.

I'm pretty surprised that changing the correct new Req_fuel value does not do the trick, as with the old Req_fuel value, I had the VE (and thus AFR) pressty much setup. Is this still normal ?

Also, this morning, I tested the pressure in my rail and I got 2.9 bar at idle, up to 3.1 bar when the throttle was opene (going down to 2.9 in cruising kPa bins). Some fluctuation is certainly normal, but could the .1 bar between 2.9 and 3 be enough to mess my AFR so the EGOcorr has to go down to 80-90%?? In other words: How sensitive to fuel pressure is the flowrate of the injectors?

Any input most welcome! Tanks, SÃ©b

detcollector · Post by **detcollector** » Wed Mar 22, 2006 7:39 am

Fuel pressure is quoted as the pressure above atmospheric and is referenced to the intake manifold.
So at full throttle opening you should get 3Bar above atmospheric. (1Bar is ~ 101KPa) If atmospheric for the day is 110KPa you will get 3.1Bar.
More interestingly:- At closed throttle idle the MAP is lower so the fuel pressure is lower. You quote 2.9 so the idle map is 3.1-2.9=.2bar lower. Or about 20KPa. So is your idle MAP 110-20 = 90KPA? I doubt it. Check your regulator and installation. This would mean you have to high a pressure at idle and cruise so you will run rich.

sebch · Post by **sebch** » Thu Mar 23, 2006 2:18 am

Hi detcollector (and the others!)

I've done some maths myself and would love if you could take a look at it and tell me what you think.

1 bar is 100 kPa (lets make it simple)
my FPR is rated at 3 bar = 300 kPA

Where I live, normal atmospheric pressure is usually 93 kPa, which would give a total of 393 kPa (and not 3.093 as your calculation would imply) with the engine off (I didn't measure that one...)

Then when my engine idles it would be 300 + 35 (that's my idle MAP) = 335 kPa. This is where I get 2.9 bar (290 kPa) instead.

When depressing the throttle a bit, there we climb to 310 kPa, meaning that the measure was made at idle MAP + 20 kPa = around 55 kPa MAPO (this seems fine)

Now would this imply that my FPR is actually maintaining the pressure around 45 kPa under its given rate, 45 being the difference between 335 kPa (expected idle pressure) and 290 kPa (actual idle pressure)?

If yes OK, I have a 300-45 = 255 kPa regulator. But then I'm back to where I was with my old FPR, so why would it inject more gas at all??

Could the fact that the vac line from the manifold to the inlet manifold is shared with the MAP sensor and the vac advance on the dizzy through a cross shaped junction be an issue? I don't think so cos' it worked fine before...

Could the fact that the previous FPR had a fuel leak through its vac line imply that my VE table is offset by the amount of pressure lost through that leak?

Thanks for your ideas! Seb

detcollector · Post by **detcollector** » Thu Mar 23, 2006 5:11 am

What are you using to measure fuel pressure?
MAP is Absolute. Fuel pressure is relative.
Your calculations are about right apart from clarifying what you used when you said fuel pressure is 2.9Bar at idle.
Max absolute fuel pressure for a 3 bar regulator ~ 393KPa as you state.
If your MAP is 35 at idle then absolute fuel pressure should be 335KPa.
If your 2.9Bar measured (290KPa) is absolute then the fuel pressure is low, it will run lean. However i think you used a relative measure wich means the absolute fuel pressure is 290+93 = 383KPa and so its high and runs very rich.
Simple checks:-
Power on a few times so the fuel pump pressurises and note the MAP (ambient) and fuel pressure. Start car and note MAP and fuel pressure. MAP and fuel pressure must change by the same amount. If not you have a problem with either the regulator or the vac connection.

sebch · Post by **sebch** » Thu Mar 23, 2006 5:20 am

Thanks fo these ideas detcollector, I'll do as you say. The device I used to check the pressure is the EFI pressure gauge I borrowed from my local garage, that has a handwritten "Jetronic pressure gauge" on it. IMO, this should display the absolute fuel pressure. Do you know of devices that don't?

sebch · Post by **sebch** » Sun Mar 26, 2006 5:49 am

OK, so it turned out to be a 3.5 bar FPR

After measuring the pressure with the engine off, that's what I got and the mix is back in its track.

Now isn't 3.5 bar (around 50 psi!!) too much for a stock non adjustble FPR ?? Can it be a sign of it being soon shot? It comes off a 3l V6 alfa engine, and after the bosch part number typed in google that it should be 3 bars...

Any hints? THanks, Seb

jonfx4com · Post by **jonfx4com** » Sun Mar 26, 2006 8:23 am

Its not too important that MAP and FP track each other exactly. It is important that the relationship is consistent and repeatable. Even if the map is not connected to the FPR at all you can map it out in VE but thats a bodge and you can end up with impossibly short idle injector pulses in some setups as it reduces the effective dynamic range of the injectors.

Jon